In order to provide for the processing of analog signals so as to derive the differences between two signals, it is oftentimes necessary to adjust the phase or time relationship between the signals at the input terminals of a differential amplifier so that the signals arrive in phase or in time-coincidence, such that an appropriate difference signal can be derived. It is common practice in instrumentation amplifiers to use two identical or balanced preamplifiers preceding a differential amplifier.
To provide for the appropriate phase change for signals that have become somewhat out of phase as is the case when certain of the analog signals are amplified, it is common practice to provide additional buffering stages which include active devices so as to alter the phase of the input signal, thereby to precondition the input signal so that it may be appropriately phased with another input signal which is also delayed. Moreover, such circuits usually must be provided with high input impedances through buffering and low output inpedances to permit further signal processing.
Nowhere is this more important than in the area of patient monitoring through electrocardiograms, in which numerous electrical signals derived from electrical contacts or electrodes located on the patient body are to be subtracted one from the other. While the subject system will be described in connection with such patient monitoring circuits, its ability to reduce the number of active components in any bank of differencing circuits permits use of the subject circuit in virtually any case in which there are multiple pairs of signals and in which one signal of a pair is to be combined with or subtracted from another signal of the pair.
For purposes of illustration, in patient monitoring, it is standard that body-carried electrodes provide analog signals corresponding to those sensed at the left arm (LA), the right arm (RA), the left leg (LL), the right leg (RL) and the chest (C). As will be appreciated, the chest position may be at one of several locations. The electrical signals which are utilized by patient monitoring instruments are the difference signals determined by pairs of the above-mentioned electrical signals, with standard pair groupings being as follows:
I (LA-RA) PA1 II (LL-RA) PA1 III (LL-LA)
These difference signals are most easily derived by analog circuitry coupled directly to receive the electrode signals.
In one typical application, there may be as many as four differential amplifiers which are connected via buffers to the four input signals LA, RA, LL and C. A further signal, RL, which refers to the signal derived from the right leg, is also utilized as a ground or neutral return. For such a system, the four differential amplifiers typically include three active devices, operational amplifiers, whereas the buffers each include at least one operational amplifier. In such a four-channel system, sixteen such active devices are utilized, which is both costly in terms of the cost of the final device, including its large, isolated power supplies, and costly also in terms of the amount of "real estate" utilized on a printed circuit board.
As will be described, the subject circuit utilizes only half the number of active devices of the prior art circuits. Thus, in the interest of being able to provide patient monitoring instrumentation with increased capability for multiple input signals, significant improvements can be afforded by reducing the number of active components in the signal conditioning circuitry. The problem is to reduce the number of active components without altering the high input impedance characteristics provided by the buffers, while at the same time permitting the filtering out of DC components without altering the low impedance output characteristic of the differential amplifiers. Moreover, it is important that all this be done while still providing output signals, which are both time-coincident and magnitude-adjusted to permit further signal processing.
The ability to accommodate multiple analog input signals with a minimum of component parts enables the production of compact instrumentation capable of simultaneously monitoring more patient parameters, thus reducing the cost of patient monitoring instrumentation needed for a given performance level beyond the cost savings afforded by the simple reduction of components. Less power is required, so the components operate cooly and last longer. Of course, for battery-operated units, it is important to reduce the overall power consumption of the circuits to increase longevity with a given battery charge. Thus, a reduction of active components not only permits less costly instrumentation, it also permits increased longevity or, concomitantly, an increased number of monitorable parameters.